1. Field of the Invention
The present invention relates to a bioelectrical impedance measuring apparatus for determining composition data of a human body.
2. Discussion of the Prior Art
The conductivity of a human body is strongly influenced by its water content. Since areas of the body which are free of fat, such as muscles and bodily fluids, contain the major part of the water content of the body, while on the other hand fat tissue has a relatively low water content, the determination of the conductivity of a body or of a body segment (or the determination of the reciprocal resistance or impedance of the body or of the body segment) allows to draw conclusions on the relative fat content, at least if further data such as body height and weight of the person are taken into account.
A method and an apparatus for bioelectrical impedance analysis are described for example in WO 97/01303. The apparatus described therein comprises eight electrodes, namely four electrodes for the feet, in each case two electrodes for contacting one foot, and four hand electrodes, in each case two electrodes for contacting one hand of the person. An alternating current is applied through two electrodes which are positioned on different limbs, and the voltage between two other electrodes is measured, which measuring electrodes are likewise positioned on different limbs. By switching over to two other current applying electrodes and voltage measuring electrodes different body segments can be examined consecutively. Furthermore, when current is injected in one hand and in one foot, and voltage is measured between the other electrode on the same hand and the other electrode on the same foot, one side of the body as a whole can be measured.
DE 195 32 760 A1 describes a bioelectrical impedance measuring apparatus having for each limb two electrodes which are attached to the skin with adhesive means, wherein on each limb one electrode is disposed more distant from the torso than the other electrode on the same limb. Current is applied through the electrodes disposed in further distance, and voltage is measured using the closer electrodes. The voltage measurement thus takes place in the current path which may lead to measuring errors since the results depend on variations in the precise positioning of the electrodes.
The bioelectrical impedance measuring apparatus described in WO 97/01303 has a standing platform for both feet on which two electrodes for a contacting the sole of foot for each foot are provided; furthermore, two hand contact bodies are provided for establishing contact with the inner surface of each hand of the user. Each hand contact body has an essentially cylindrical shape so that it may be grasped by a hand, the surface of the palm and the inner surface of the fingers so coming into contact with the surface of the hand contact body. On each hand contact body two electrodes are provided, namely one in the area of the cylinder barrel and the other on the front end face of the cylinder such that when the user grasps the hand contact body the one electrode comes into contact with the palm or the inner surface of the fingers and the other electrode comes into contact with the thumb placed on the front end face. In this manner, by establishing contact at the palm and at the thumb, to a certain degree a decoupling of the current path and of the voltage measuring path is achieved which, however, does not result in a reliable measurement of the impedance. In fact the thumb is particularly moveable so that the point where the current path and the voltage measuring path meet within the body strongly varies with the variable thumb position. Therefore, there is a strong dependence of the measured impedance value on the thumb position.